For high-volume (for example, many thousands) production of metal parts (workpieces) on milling machines and other machine tools, it can be practical to employ expensive quick-acting apparatus to hold down the workpieces that are being machined. Such apparatus is typically hydraulic. At the other end of the scale, when only a few metal parts are to be milled or otherwise machined, it is practical to use ordinary bolts and nuts to hold down the workpieces.
However, it is common for small production runs to occur. For example, a few hundreds or thousands of parts may be made identical to each other, as distinguished from the many thousands that would justify expensive quick-acting hydraulic hold-down apparatus. In production runs of this type, speed in changing from one part to the other is of paramount importance. Further, even during machining of the same part, it is frequently necessary to change the position of the hold-down apparatus in order for the entire machining of such part to be completed.
It is emphasized that when no workpiece is present, the hold-down apparatus should occupy as little space as possible--should not get in the way. However, it is also important in a production run that the hold-down apparatus will stay in the same place for part after part without any necessity for measuring, adjustment or positioning work by the machinist or operator.
In addition to the above, the cost of the hold-down apparatus is of distinct significance. If the hold-down apparatus is not relatively inexpensive, as well as being simple, easy and fast to operate, it will probably not achieve commercial success. It is pointed out that some workpieces require a substantial number of hold-down elements, this being another factor relative to the overall cost to the machine shop or manufacturer.
A further important factor is that the hold-down apparatus should not be such as to become clogged by chips and granules that fly off the workpiece during machining, cutting and other operations. It is not desired that the operator be required to stop production and blow out granules, etc., at frequent intervals.
Another factor to be taken into account is tolerances relative to the thickness of the part being manufactured. It should be understood that the workpieces will vary slightly in thickness from part to part.